Organismal anti-viral defenses are designed to detect virus and activate virus-lethal mechanisms. Positive feedback loops up-regulate virus- detection mechanisms in an effort to outpace the exponential spread of virus. Virus detection mechanisms include responses to double-stranded RNA, which induce (2'-5') oligoadenylate synthetase (OAS) activity and interferon (IFN) mRNA. Additionally, antigenic viral peptides bind to intracellular MHC class I molecules and stimulate their export to the cell surface where they can be detected by the cytotoxic T lymphocytes (CTL) of the immune system. In wild-type cells, mRNAs encoding both OAS and MHC molecules are up-regulated by treatment with interferon, thus increasing cellular responsiveness to dsRNA and antigenic peptide. Responsiveness to IFN itself may be autogenic: interferon also induces mRNA encoding the putative interferon-responsive transcription factor, IRF-1. By selecting for resistance to lysis by antigen-specific CTL, six independent MHClow cell mutants were isolated in which mRNAs encoding OAS, MHC and beta2m are coordinately depressed. Cells in this novel class of MHClow mutants respond poorly to interferon stimulation, and are extremely sensitive to virus infection. The mutation is recessive, and can be complemented in trans through somatic cell hybridization. These genetic studies suggest the existence of previously undocumented trans-acting regulatory genes governing both basal and IFN-inducible expression of several pathways of virus-resistance. These regulatory genes are termed mav (for Mhc and anti-viral). The objectives of this research are to define the cis- and trans-acting genetic factors involved in the coordinate regulation of these virus resistance pathways. Wild-type genes can complement the defects in mav mutants. Therefore, mav+ genes will be isolated through selection for DNA- dependent reversion. Gene chimera and transcription reporter methods will identify cis-acting sequence elements which respond to mav functions. Finally, parallel studies will investigate the role of OAS and IRF-1 in regulating MHC expression by over-expressing these genes in mav mutants.